1. Technical Field of the Invention
The present invention generally relates to a method for fabricating a dielectric material that has an ultralow dielectric constant (or ultralow-k) associated therewith. More particularly, the present invention relates to an improved method for fabricating a thermally stable ultralow-k film for use as an intralevel or interlevel dielectric in an ultra-large-scale integration (xe2x80x9cULSIxe2x80x9d) back-end-of-the-line (xe2x80x9cBEOLxe2x80x9d) wiring structure.
2. Description of the Prior Art
The continuous shrinking in dimensions of electronic devices utilized in ULSI circuits in recent years has resulted in increasing the resistance of the BEOL metallization as well as increasing the capacitance of the intralayer and interlayer dielectric. This combined effect increases signal delays in ULSI electronic devices. In order to improve the switching performance of future ULSI circuits, low dielectric constant (k) insulators and particularly those with k significantly lower than silicon oxide are needed to reduce the capacitances. Dielectric materials (i.e., dielectrics) that have low-k values have been commercially available. For instance, one such material is polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d), which has a k value of 2.0. However, these dielectric materials are not thermally stable when exposed to temperatures above 300xcx9c350xc2x0 C. Integration of these dielectrics in ULSI chips requires a thermal stability of at least 400xc2x0 C. Consequently, these dielectrics are rendered useless during integration.
The low-k materials that have been considered for applications in ULSI devices include polymers containing Si, C, O, such as methylsiloxane, methylsilsesquioxanes, and other organic and inorganic polymers. For instance, a paper (N. Hacker et al. xe2x80x9cProperties of new low dielectric constant spin-on silicon oxide based dielectrics.xe2x80x9d Mat. Res. Soc. Symp. Proc. 476 (1997):25) described materials that appear to satisfy the thermal stability requirement, even though some of these materials propagate cracks easily when reaching thicknesses needed for integration in the interconnect structure when films are prepared by a spin-on technique. Furthermore, the precursor materials are high cost and prohibitive for use in mass production. In contrast to this, most of the fabrication steps of very-large-scale-integration (xe2x80x9cVLSIxe2x80x9d) and ULSI chips are carried out by plasma enhanced chemical or physical vapor deposition techniques. The ability to fabricate a low-k material by a plasma enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d) technique using readily available processing equipment will simplify the material""s integration in the manufacturing process, reduce manufacturing cost, and create less hazardous waste. A co-pending application (Hydrogenated Oxidized Silicon Carbon Material, U.S. Ser. No. 09/107,567, filed on Jun. 19, 1998) assigned to the common assignee of the present invention and incorporated herein by reference in its entirety, described an ultralow dielectric constant material, consisting of Si, C, O and H atoms, having a dielectric constant not more than 3.6, and exhibiting very low crack propagation velocities.
Another co-pending application (Multiphase Low Dielectric Constant Material and Method of Deposition, U.S. Ser. No. 09/320,495, filed on May 16, 1999) assigned to the common assignee of the present invention and incorporated herein by reference in its entirety, described a dual-phase material, consisting of a matrix composed of Si, C, O, and H atoms, a phase composed of mainly C and H atoms, and having a dielectric constant of not more than 3.2. It should be noted that continued reduction of the dielectric constant of such materials will further improve the performance of electronic devices incorporating such dielectrics.
In view of the foregoing, there is a continued need for developing a dielectric material that has a dielectric constant of not more than about 2.8 and inhibits cracking.
It is therefore an object of the present invention to provide an improved method for fabricating an ultralow dielectric constant material having a dielectric constant of not more than about 2.8. More preferably, the dielectric constant for the ultralow-k material is in a range of about 1.5 to about 2.5, and most preferably, the dielectric constant is in a range of about 2.0 to about 2.25. It should be noted that all dielectric constants are relative to a vacuum unless otherwise specified.
It is another object of the present invention to provide an improved method for fabricating an ultralow dielectric constant material comprising Si, C, O and H atoms from a mixture of at least two precursors, wherein a first precursor is selected from molecules with ring structures comprising SiCOH components and a second precursor is an organic molecule selected from the group consisting of molecules with ring structures.
It is a further object of the present invention to provide an improved method for fabricating an ultralow dielectric constant film in a parallel plate plasma enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d) reactor.
It is yet a further object of the present invention to provide an improved method for fabricating an ultralow dielectric constant material by depositing a film on a substrate in the presence Of CO2 or CO2 and O2, thereby improving uniformity of the deposited film deposited and stabilizing the plasma within the PECVD reactor.
It is still a further object of the present invention to provide an improved method for fabricating an ultralow dielectric constant material for use in electronic structures as an intralevel or interlevel dielectric in a back-end-of-the-line (xe2x80x9cBEOLxe2x80x9d) interconnect structure.
It is yet another object of the present invention to provide a thermally stable ultralow dielectric constant material that has low internal stresses and a dielectric constant of not higher than about 2.8. More preferably, the dielectric constant for the ultralow-k material is in a range of about 1.5 to about 2.5 and, most preferably, the dielectric constant is in a range of about 2.0 to about 2.25.
In accordance with the present invention, there is provided an improved method for fabricating a thermally stable dielectric material that has a matrix comprising Si, C, O, and H atoms and an atomic level nanoporosity. In a preferred embodiment, the dielectric material has a matrix that consists essentially of Si, C, O, and H. The present invention further provides an improved method for fabricating the dielectric material by reacting a first precursor gas comprising atoms of Si, C, O, and H and at least a second precursor gas comprising atoms of C, H, and optionally O, F and N in a plasma enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d) reactor. The present invention yet further provides for mixing the first precursor gas with CO2, or mixing the first and second precursor gases with CO2 and O2, thereby stabilizing the plasma in the PECVD reactor and improving the uniformity of the film deposited on the substrate.
In a preferred embodiment, there is provided a method for fabricating a thermally stable ultralow dielectric constant (ultralow-k) film comprising the steps of: providing a plasma enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d) reactor; positioning an electronic structure (i.e., substrate) in the reactor; flowing a first precursor gas comprising atoms of Si, C, O, and H into the reactor; flowing a second precursor gas mixture comprising atoms of C, H and optionally O, F and N into the reactor; and depositing an ultralow-k film on the substrate in the presence of CO2 or CO2 and O2. Preferably, the first precursor is selected from molecules with ring structures comprising SiCOH components such as 1, 3, 5, 7-tetramethylcyclotetrasiloxane (xe2x80x9cTMCTSxe2x80x9d or xe2x80x9cC4H16O4Si4xe2x80x9d). The second precursor may be an organic molecule selected from the group consisting of molecules with ring structures, preferably with more than one ring present in the molecule. Especially useful, are species containing fused rings, at least one of which contains a heteroatom, preferentially oxygen. Of these species, the most suitable are those that include a ring of a size that imparts significant ring strain, namely rings of 3 or 4 atoms and/or 7 or more atoms. Particularly attractive, are members of a class of compounds known as oxabicyclics, such as cyclopentene oxide (xe2x80x9cCPOxe2x80x9d or xe2x80x9cC5H8Oxe2x80x9d).
Optionally, the deposited film of the present invention can be heat treated at a temperature of not less than about 300xc2x0 C. for a time period of at least about 0.25 hour. The method may further comprise the step of providing a parallel plate reactor, which has a conductive area of a substrate chuck between about 300 cm2 and about 700 cm2, and a gap between the substrate and a top electrode between about 1 cm and about 10 cm. A high frequency RF power is applied to one of the electrodes at a frequency between about 12 MHZ and about 15 MHZ. Optionally, an additional low frequency power can be applied to one of the electrodes. The heat-treating step may further be conducted at a temperature not higher than about 300xc2x0 C. for a first time period and then at a temperature not lower than about 380xc2x0 C. for a second time period, the second time period being longer than the first time period. The second time period may be at least about 10 times the first time period.
The deposition step for the ultralow dielectric constant film of the present invention may further comprise the steps of: setting the substrate temperature at between about 25xc2x0 C. and about 400xc2x0 C.; setting the high frequency RF power density at between about 0.05 W/cm2 and about 2.0 W/cm2; setting the first precursor flow rate at between about 5 sccm and about 1000 sccm; setting the flow rate of the second precursor between about 5 sccm and about 1000 sccm; setting the reactor pressure at a pressure between about 50 mTorr and about 5000 mTorr; and setting the high frequency RF power between about 15 W and about 500 W. Optionally, an ultralow frequency power may be added to the plasma between about 10 W and about 300 W. When the conductive area of the substrate chuck is changed by a factor of X, the RF power applied to the substrate chuck is also changed by a factor of X.
In another preferred embodiment, there is provided a method for fabricating an ultralow-k film comprising the steps of: providing a parallel plate type chemical vapor deposition reactor that has plasma enhancement; positioning a pre-processed wafer on a substrate chuck which has a conductive area of between about 300 cm2 and about 700 cm2 and maintaining a gap between the wafer and a top electrode between about 1 cm and about 10 cm; flowing a first precursor gas comprising cyclic siloxane molecules into the reactor; flowing at least a second precursor gas comprising organic molecules with ring structures including C, H and O atoms; and depositing an ultralow-k film on the wafer in the presence of CO2 or CO2 and O2. The process may further comprise the step of heat-treating the film after the deposition step at a temperature of not less than about 300xc2x0 C. for at least about 0.25 hour. The process may further comprise the step of applying a RF power to the wafer. The heat-treating step may further be conducted at a temperature of not higher than about 300xc2x0 C. for a first time period and then at a temperature not lower than about 380xc2x0 C. for a second time period, the second time period being longer than the first time period. The second time period may be at least about 10 times the first time period.
The cyclic siloxane precursor utilized can be tetramethylcyclotetrasiloxane (xe2x80x9cTMCTSxe2x80x9d) and the organic precursor can be cyclopentene oxide (xe2x80x9cCPOxe2x80x9d). The deposition step for the ultralow-k film may further comprise the steps of: setting the wafer temperature at between about 25xc2x0 C. and about 400xc2x0 C.; setting a RF power density at between about 0.05 W/cm2 and about 2.0 W/cm2; setting the flow rate of the cyclic siloxane between about 5 sccm and about 1000 sccm; setting the flow rate of the organic precursor between about 5 sccm and about 1000 sccm; and setting the pressure reactor at between about 50 mTorr and about 5000 mTorr. Additionally, the deposition step may further comprise setting a flow ratio of cyclopentene oxide to tetramethylcyclotetrasiloxane to between about 0.1 and about 0.7, preferably between 0.2 and 0.4. The conductive area of the substrate chuck can be changed by a factor X, which leads to a change in RF power by the same factor X.
In still another preferred embodiment, there is provided a method for fabricating a thermally stable ultralow-k dielectric film comprising the steps of: providing a plasma enhanced chemical vapor deposition reactor of a parallel plate type; positioning a wafer on a substrate chuck that has a conductive area between about 300 cm2 and about 700 cm2 and maintaining a gap between the wafer and a top electrode between about 1 cm and about 10 cm; flowing a precursor gas mixture of a cyclic siloxane with a cyclic organic molecule into the reactor over the wafer, which is kept at a temperature between about room temperature and about 400xc2x0 C. and preferably between about 60xc2x0 C. and about 200xc2x0 C., at a total flow rate between about 25 sccm and about 500 sccm while keeping the reactor pressure at between about 100 mTorr and about 5000 mTorr; depositing a dielectric film on the wafer under a RF power density between about 0.25 W/cm2 and about 0.8 W/cm2 in the presence of CO2 or CO2 and O2; and annealing the ultralow-k film at a temperature of not less than about 300xc2x0 C. for at least about 0.25 hour. The inventive method may further comprise the step of annealing the film at a temperature of not higher than about 300xc2x0 C. for a first time period and then at a temperature not lower than about 380xc2x0 C. for a second time period, wherein the second time period is longer than the first time period. The second time period may be set at least about 10 times the first time period. The cyclic siloxane precursor can be tetramethylcyclotetrasiloxane (xe2x80x9cTMCTSxe2x80x9d) and the cyclic organic precursor can be cyclopentene oxide (xe2x80x9cCPOxe2x80x9d).